Hi, we wnat to migrate to lightning but one of our issues is the use of javascript and validation. Can anyone show be how to get a button or action in Lighting for the following? I have read articles, but we validate fields with javascript and also determine what kind of record to create based on picklist selection. Please help if possible, thanks!

if('{!Opportunity.Related_Contract__c}'!=''&&'{!Opportunity.Type_of_Contract__c}' != 'Addendum'){ 
alert("Sorry, a Contract record has already been created for this opportunity. You can view the Contract by clicking on the Related Contract link in the Contract/Amendment Details section."); 
}
else if('{!Opportunity.Related_Amendment__c}'!=''&&'{!Opportunity.Type_of_Contract__c}' == 'Addendum'){ 
alert("Sorry, an Amendment record has already been created for this opportunity. You can view the Amendment by clicking on the Related Amendment link in the Contract/Amendment Details  section."); 
}

else if('{!Opportunity.Type_of_Contract__c}' == ''){
alert("Please enter a Type of Contract before requesting a Contract or Addendum."); 
}
else if('{!Opportunity.Contact__c}' == ''){
alert("Please enter a Contract Contact before requesting a Contract or Addendum."); 
}
else if(!('{!Opportunity.Type_of_Contract__c}'=='')&&'{!Opportunity.Has_Banker_PPE_Service__c}'>0&&('{!Opportunity.Cost_Per_Additional_User__c}'==''||'{!Opportunity.Users__c}'=='')){
alert("This opportunity has a Banker PPE service. Before requesting a Contract or Addendum, please provide the number of Users and the Cost per Additional User in the Contract Details section.");
} 
else if('{!Opportunity.Type_of_Contract__c}' == 'Addendum'){
window.open ( "/a4u/e?saveURL={!Opportunity.Id}&retURL=%2F{!Opportunity.Id}&00N38000003rUFs={!Opportunity.Name}&CF00N38000003rUFn={!Opportunity.Name}&CF00N38000003rUFn_lkid={!Opportunity.Id}&CF00N38000003rUFd={!Opportunity.Related_Contract__c}&CF00N38000003rUFd_lkid={!Opportunity.Related_ContractId__c}","_top");
}
else{
window.open ( "/800/e?saveURL={!Opportunity.Id}&retURL=%2F{!Opportunity.Id}&00N38000003Hm3a={!Opportunity.Type_of_Contract__c}&Name={!Opportunity.Name}&ctrc40={!Opportunity.Contract_Term__c}&00N38000003Hj9L={!Opportunity.Renewal_Term_months__c}&00N38000003Hm4E=1&CF00N50000002bj33={!Opportunity.Name}&CF00N50000002bj33_lkid={!Opportunity.Id}&ctrc15=New&ctrc7={!URLENCODE(Opportunity.Account_Name_TEXT__c) }&ctrc7_lkid={!Opportunity.AccountId}","_top");
}